1. Field of the Invention
The present invention relates to chemical mechanical polishing processes and in particular to detecting scratching of substrates during chemical mechanical polishing. Still more particularly, the present invention relates to employing acoustic spectral analysis during chemical mechanical polishing to detect abnormal vibrations indicating scratching of a substrate being polished.
2. Description of the Prior Art
Chemical mechanical polishing (CMP) processes are commonly employed in place of wet or dry etching to remove material from a substrate during semiconductor device fabrication. Such chemical mechanical polishing typically involves saturating a polyester polishing pad mounted on a polishing platen with an abrasive, chemical etchant polishing slurry, then lowering a chuck holding a wafer until the wafer surface to be polished contacts the saturated surface of the polishing pad. The polishing platen and pad are then rotated, usually with the chuck and wafer being independently rotated, while the wafer surface and the polishing pad are in contact until a desired amount of material has been removed.
Occasionally a scratch-causing particlexe2x80x94such as a metal fragment broken off from the substrate being polishedxe2x80x94will become trapped on or within the polishing pad during chemical mechanical polishing. Since device features may be destroyed or proper feature spacing precluded by scratching, such scratch-causing particles usually ruin the wafer, or at least a significant portion of the integrated circuit die on the wafer.
Current ex-situ control of chemical mechanical polishing processes employ visual inspection and/or optical testing of the polished wafers to detect scratching. Due to batch processing considerations and other factors introducing delay between the chemical mechanical polishing and analysis of the polished wafers for scratching, a large number of wafers may become scratched by a particle embedded within the polishing pad before such scratching is detected. Typically a minimum of 25 wafersxe2x80x94and often as many as 400 wafers or morexe2x80x94may be damaged or destroyed before detection of the problem and replacement of the polishing pad.
It would be desirable, therefore, to provide an in-situ mechanism for detecting scratching of wafers during chemical mechanical polishing.
Acoustic emission samples for a chemical mechanical polishing process are acquired and analyzed using a Fourier transform to detect wafer vibrations characteristic of scratching. When excess noise levels are detected at frequencies or within frequency bands being monitored, the polishing process is halted and an alarm is generated for the operator. Such in-situ detection minimizes damage to the wafer being polished and limits the damage to a single wafer rather than a run of wafers. Polish endpoint detection may be integrated within the scratch detection mechanism.